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Volume 72, Issue 11, 01 June 1980

Monomer–eximer dynamics in spread monolayers. I. Lateral diffusion of pyrene dodecanoic acid at the air–water interface
View Description Hide DescriptionIn this paper, we present a combined experimental and theoretical study of monomer–eximer dynamics in spread monolayers. The system studied here is a spread monolayer of 12‐(1‐pyrene) dodecanoic acid(PDA) and oleic acid at the air–water interface of a neutral aqueous solution. The microscopic kinetic behavior of the PDA probe in the monolayer was monitored through the eximer–monomer, steady‐state photoexcitation of the pyrene moiety. The steady‐state ratio of eximer to monomer intensities, I _{2}/I _{1}, was found to depend linearly on the mole fraction of PDA in oleic acid over the concentration range studied. These data were interpreted theoretically by constructing a reaction‐dynamic model to describe the lateral diffusion and subsequent interaction of the pyrene species. Using analytical results from the theory of random walks on lattices with traps, and from data from Monte Carlo simulations, an estimate is obtained for the two‐dimensional diffusion coefficient of the pyrene probe in the monolayer; this estimate is 1.7×10^{−6} cm^{2}/sec.

Raman spectra of liquid n‐alkanes. II. Longitudinal acoustic modes and the g a u c h e–t r a n s energy difference
View Description Hide DescriptionLow‐frequency bands in the Raman spectra of the liquid n‐alkanes, n=9–20, have been identified with extended and certain nearly extended rotameric forms of these molecules. The bands are associated with the intense longitudinal‐acoustic‐mode (LAM‐1) band that appears in the Raman spectra of crystalline n‐alkanes. For liquids, the intensities of these bands are highly temperature dependent. This dependence was measured for the n=11–14 n‐alkanes and was used to determine an average value for ΔE, the energy difference between the t r a n s and g a u c h e states of the CC bond. The value determined for ΔE, 508±50 cal/mole, is very near the value commonly assumed in statistical calculations based on the rotational isomeric model. No evidence for local order was found even at temperatures just above the melting point of n‐C_{13}H_{28} and n‐C_{14}H_{30}.

The temperature dependence of the diamagnetism of liquids
View Description Hide DescriptionWe have measured the temperature dependence of the mass magnetic susceptibility of a representative group of liquids using a superconducting susceptometer. This study was undertaken to test the hypothesis that the known temperature dependence of the mass susceptibility of water [(13.35±0.03)×10^{−5} K^{−1}] is due to thermally induced changes in its hydrogen bonding structure. The liquids we have measured in increasing order of their temperature dependent susceptibility (given in parentheses in units of 10^{−5} K^{−1}) are: carbon tetrachloride (1.37±0.30); chloroform (2.90±0.35); carbon disulfide (3.65±0.39); acetone (3.93±0.39); methanol (4.02±0.34); and acetic acid (4.02±0.35). These results indicate there is no simple quantitative correlation between hydrogen bonding and the temperature dependence of the diamagnetism of a liquid. Our results can be expressed in the form of a virial expansion: χ(T) =χ_{0}+χ_{1}ρ_{ A }(T), with the virial coefficients χ_{0} and χ_{1} being independent of temperature. Thus the observed temperature dependence in liquids other than water may be due entirely to changes in molar volume brought about by changes in density with temperature. This hypothesis could be tested by measurements at various fixed temperatures of the pressure dependence of the mass susceptibility of these liquids.

Li_{2} and Na_{2} ^{3}Σ_{ g } ^{+}–^{3}Σ_{ u } ^{+} excimer emission
View Description Hide DescriptionA b i n i t i o calculations show the ^{3}Σ_{ g } ^{+}–^{3}Σ_{ u } ^{+} in Li_{2} and Na_{2} dimers to be primarily a near‐infrared continuum with respective v′=0 lifetimes of 62 and 15 nsec. The peak stimulated emission cross section for v′=0 is 4.5×10^{−−6} at 1.3μ for Li_{2} and 1.8×10^{−15} cm^{2} at 0.83μ for Na_{2}. These calculations suggest a tunable high gain, near‐infrared laser excimer if the ^{3}Σ_{ g } ^{+} state can be populated sufficiently rapidly.

Manganese hexacyanomanganate: Magnetic interactions via cyanide in a mixed valence Prussian blue type compound
View Description Hide DescriptionA Prussian blue type compound of the stoichiometric composition Mn(CN)_{3}⋅xH_{2}O (x=0.57) is formed as the final product of hydrolysis of Na_{3}[Mn(CN)_{6}] in perchloric acid. IR and visible spectra as well as structural and magnetic data show that it is a mixed valence compound consisting of sixfold N‐coordinated Mn(II) and sixfold C‐coordinated Mn(IV) in a cubic face‐centered lattice. The compound exhibits ferrimagnetism with a Curie temperature of 48.7 K. A molecular field treatment assuming two collinear spin sublattices provides an adequate description of the observed magnetic behavior. The calculation of exchange integrals, which was based on a Heisenberg model, reveals that the magnetic ordering is almost exclusively induced by an antiferromagnetic interaction between Mn(II) and Mn(IV) ions. This interaction is described in terms of a super exchange involving cyanide as bridging ligand.

Photoelectron spectroscopy of AgCl, AgBr, and AgI vapors
View Description Hide DescriptionHe I photoelectron spectra of AgCl, AgBr and AgI vapors have been obtained which differ significantly from earlier work. In each instance, the characteristic features of the diatomic molecule are prominent. The spectral features separate into a valence region, predominantly halogen p‐like, and a deeper region, predominantly of Ag 4d character. The latter is split by spin–orbit and ligand field interactions, which are parametrized from the experimental data. Relativistic calculations of the X _{α}–DVM–SCC type have been performed for these species. At the transition state level, they agree very well with the experimental peak positions. Nonrelativistic calculations of this type have been performed for CuCl and cyclic Cu_{3}Cl_{3}. Unlike the AgX species, the CuCl and Cu_{3}Cl_{3} exhibit strong mixing of metal d and halogen p orbitals for the uppermost occupied orbital, and other Cu 3d‐like orbitals above the Cl 3p‐like orbitals. It is suggested that the occurrence of Cu 3d orbitals in the valence region may play a role in the anomalous diagmagnetic signal and large conductivity changes of CuCl condensed from the vapor.

The dual melting curves and metastability of carbon tetrachloride
View Description Hide DescriptionCarbon tetrachloride has three known solid phases at atmospheric pressure: Ia (face‐centered cubic), Ib (rhombohedral), and II (monoclinic). Both Ia and Ib melt directly at temperatures some 5 K apart. These phase changes have been traced as a function of hydrostaticpressure up to 350 MPa. Between atmospheric pressure and 100 MPa, CCl_{4} has dual melting curves; one for Ia, and a few degrees higher, one for Ib. The two curves diverge with increasing pressure. Above 100 MPa it was not possible to detect the Ia phase. There appears to be no Ia–Ib‐liquid triple point. The metastability associated with these phases is discussed.

The chemical dynamics of the reactions of O(^{3} P) with saturated hydrocarbons. I. Experiment
View Description Hide DescriptionMolecular beam‐laser induced fluorescence experiments have probed the nascent internal state distributions and excitation functions of OH formed in the technologically important reactions O(^{3} P)+RH→OH+R⋅. RH is a saturated hydrocarbon and R⋅ is an alkyl radical. A variety of RH have been investigated corresponding to abstraction of primary, secondary, and tertiary hydrogens. The OH rotational state distribution is nearly identical for all RH and decrease rapidly from its peak at the lowest rotational level. This demonstrates that reaction occurs when O(^{3} P) is collinear to a C–H bond in the hydrocarbons. The vibrational state distribution of OH depends markedly on the type of hydrogen abstracted, with vibrational excitation increasing dramatically across the series primary to secondary to tertiary. This is interpreted as a shift from a repulsive towards a more attractive surface across the series. Partitioning into the OH spin doublets shows that these reactions are midway between the adiabatic and diabatic limits with respect to the spin–orbit surfaces. Excitation functions measure the dynamic thresholds, and are in good agreement with activation energies obtained from thermal rate constants. The excitation functions for v=1 OH exhibit a sharp decrease at energies considerably above threshold. This suggests that excitation of internal modes of R⋅ occurs only at high collisions energies. All of these results are interpreted qualitatively in terms of a simple, but general, triatomic model for the interaction of O(^{3} P) with RH, i.e., where R⋅ can be considered a structureless particle.

The chemical dynamics of the reactions of O(^{3} P) with saturated hydrocarbons. II. Theoretical model
View Description Hide DescriptionQuasiclassical trajectory calculations on modelpotential surfaces are used to explain the experimental results of the previous paper on the O(^{3} P) abstraction reactions with saturated hydrocarbons. OH rotational and vibrational state distributions and excitation functions are calculated assuming a triatomic R–H–O model for the reactions. A simple empirical LEPS potential surface is constructed to fit the experimental data for only reaction with a secondary hydrocarbon. The potential surfaces for other hydrocarbons are identical except for known variations in the C–H bond strength. The excellent agreement of these calculations with the experimental results for all hydrocarbons justifies the triatomic model, and demonstrates the predictive nature of the O(^{3} P)+ hydrocarbon model surfaces.

Pyroelectric Ba(NO_{2})_{2}⋅H_{2}O: Room temperature crystal structure
View Description Hide DescriptionBarium nitrite monohydrate, Ba(NO_{2})_{2}⋅H_{2}O, is strongly pyroelectric at room temperature and crystallizes in the hexagonal system with space group P6_{5}, or the enantiomorphous P6_{1}, and with six formulas in the unit cell. The lattice constants at 298 K are a=7.074 90±0.000 03 and c=17.890 87±0.000 12 Å (λCuKα_{1}=1.540598 Å). The integrated intensities of most reflections within a quadrant of reciprocal space having radius (sinϑ)/λ?1.15 or 1.08 Å^{−1} were measured on two different crystals, each with different diffractometer, resulting in 3218 F m e a s for one set and 3671 F m e a s for the second set of independent structure factors. The crystal structure was solved from Patterson and Fourier series, and refined by the method of least squares. The final agreement factor R=0.026 for the first set, 0.018 for the second set of F m e a s. The two sets of F m e a s and the derived structural parameters are compared using normal probability plot analysis. The two NO^{−} _{2} ions have a mean N–O distance of 1.246±0.002 Å and O–N–O angle of 114.2±0.2°. The Ba^{2+} ion is coordinated by nine oxygen and one nitrogen atoms at an average distance of 2.899 Å. The water molecule has a weighted mean O–H distance of 1.04±0.03 Å and H–O–H angle of 102.7°±4.3°: it is hydrogen bonded to two independent NO^{−} _{2} ions by an O–H⋅⋅⋅O bond of 2.878 Å and an O–H⋅⋅⋅N bond of 2.951 Å.

Interaction of molecules with electromagnetic fields. I. Classical particles and fields
View Description Hide DescriptionClassical models of electrons and nuclei are developed which behave surprisingly like their quantum mechanical counterparts. The particles are finite in size and have both electrical charge density and current density. Classical spin results from the angular momentum of the particles rotating about their ’’center of mass’’. Internal electromagnetic fields are produced by the motion of the charged particles. These internal fields have dynamical properties of their own and produce the interaction between the particles. In this paper, the internal fields are derived through the second order of retardation or terms in 1/c ^{2}. Since higher order terms in the internal fields are not included, no difficulties are encountered with respect to the classical particles radiating. This same result could also be obtained by using equal parts of the retarded and advanced potentials. The Lagrangian, the (canonical, mechanical, and angular) momenta, the energy, and the Hamiltonian are derived in a gauge invariant manner for a ’’molecular system’’ of particles in the presence of an arbitrary (time and space varying) external electromagnetic field. The classical Hamiltonian corresponds term‐by‐term with the comparable quantum mechanical Breit–Pauli Hamiltonian. The correspondence includes the spin–orbit interaction terms, the Darwin terms (usually attributed to zitterbewegung), and the nuclear quadrupole terms (usually omitted from molecular Hamiltonians). A detailed comparison of classical and quantum mechanical dynamical properties will be given in a subsequent paper. Delta functions occur in both the classical and the quantum mechanical Hamiltonians to approximate the effect of overlapping when particles come close together. Otherwise, overlapping is generally not considered in either the classical or quantum treatments. In deriving the kinetic energy of a particle rotating about its center of mass, there was discovered a new relation which the rate of change of intrinsic spin (in the laboratory reference frame) must satisfy in order that the Lagrangian should completely describe the motion of the particle. This led to the discovery of a condition of compatibility between the dynamical properties of classical and quantum mechanical systems. This condition then led to a novel derivation of the relationship between the explicit expressions for the multipole moments of a particle in its laboratory reference frame (the o b s e r v e d moments) and the multipole moments in the particle’s rest frame (the i n t r i n s i c moments). It is assumed that an electron has only an i n t r i n s i cmagnetic dipole moment whereas a nucleus has both an i n t r i n s i cmagnetic dipole moment and an i n t r i n s i c electric quadrupole moment. The condition of compatibility also led to the derivation of canonical equations of change and generalized Poisson brackets, in both of which the spins are included as basic dynamical variables. The generalized Poisson brackets of the positions, the canonical momenta, and the spins exhibit the same forms as the commutation relations of their quantum mechanical counterparts. The (time and space varying) functions of the fields are expanded in Taylor series which have many of the features of Cartesian tensorial representations. If these functions were expanded in terms of spherical harmonics, the formulation would have been much more complicated.

Photodissociation of vibrationally excited ozone
View Description Hide DescriptionA two laser photodissociation technique has been used to measure the effect of ozone vibrational excitation on the cross section for photodissociative production of O(^{1} D) at several UV wavelengths in the Hartley band. Below the 310 nm energy threshold for the process, O_{3}+hν O_{2}(^{1}Δ_{ g })+O(^{1} D), vibrational excitation of O_{3} was found to increase the cross section for production of O(^{1} D) by nearly two orders of magnitude. Substantial increases in both the O_{3}photoabsorption cross section and the O(^{1} D) quantum yield were observed. Between the 310 nm threshold and the 255 nm absorption peak of the Hartley band, the effect of vibrational excitation was observed to decrease with increasing UVphotonenergy. The quantum yield of O(^{1} D) for photodissociation of unexcited, room temperature (300 °K) ozone was also measured to be Φ_{0}=0.15±0.07 at λ=314.5 nm.

Polarizability calculations with the SCF method. III. Ethylene, butadiene, and hexatriene
View Description Hide DescriptionWe evaluate the nonlinear third‐order electric susceptibilities of ethylene, t r a n s‐butadiene, and t r a n s‐hexatriene by combining the Pariser–Parr–Pople method and the extended Hückel method. It follows from the nature of the calculations that the theoretical results are quite sensitive to approximations since the final results are obtained from various positive and negative contributions. If we allow for a possible 10% error in each contribution, then our result and the corresponding error margins are γ=−0.03±0.03 for ethylene, γ=1.35±0.75 for t r a n s‐butadiene, and γ=9.92 ±5 for t r a n s‐hexatriene, all values are expressed in terms of 10^{−36} esu. We feel that these values are consistent with experimental results obtained by Ward and Elliott.

Dielectric relaxation spectrum of undiluted poly(4‐chlorostyrene), T≳T _{ g }
View Description Hide DescriptionDielectric relaxationcharacteristics of undiluted, atactic poly(4‐chlorostyrene), P4CS, have been determined at temperatures 406 K⩽T⩽446 K from measurements made at frequencies 0.2 Hz⩽f⩽0.2 MHz. After effects of electrical conductivity are subtracted, it is found that the normalized complex dielectric constantK ^{*}=K′−i K″ can be represented quantitatively by the Havriliak–Negami (H–N) equation K ^{*}=[1+(iωτ_{0})^{1‐α}]^{−β}, 0⩽α, β⩽1, except for a small, high frequency tail that appears in measurements made near the glass transition temperature, T _{ g }. The parameter β is nearly constant, and α depends linearly on log τ_{0}, where τ_{0} is a characteristicrelaxation time. The parameters α and β extrapolate through values obtained from published data from P4CS solutions, and extrapolation to α=0 yields a value of τ_{0} which compares favorably with a published value for crankshaft motions of an equivalent isolated chain segment. These observations suggest that β may characterize effects of chain connectivity and α may describe effects of interactions of the surroundings with the chain. Experimental results are compared with alternative empirical and model‐based representations of dielectric relaxation in polymers.

The temperature dependence of hydrogen abstraction reactions: F+HCl, F+HBr, F+DBr, and F+HI
View Description Hide DescriptionA laser photolysis/infrared fluorescence technique has been used to study the temperature dependence of the F+HCl, F+HBr, F+DBr, and F+HI absolute rate constants in the range from 195 to 373 °K. Multiphoton dissociation of SF_{6} with a TEA CO_{2} laser was used to provide a transient concentration of fluorine atoms. Their subsequent reaction with hydrogen donors was monitored by observing the appearance rate of HF(v?1) through its infrared fluorescence. The observed rate constants show a marked nonlinear Arrhenius behavior which is consistent with a model in which reaction takes place both vai direct attack by the fluorine atom at the hydrogen end of HX and via formation of a collision complex FXH which rearranges to FHX and then dissociates to HF and X products.

Rotational tunneling in methane (CD_{4}): Isotope effect
View Description Hide DescriptionWell‐defined tunneling lines in the μeV range have been observed in CD_{4} at T=4 K by high resolution neutron scattering. The observed change of tunneling energies by about a factor 50 upon deuteration is in good accord with a theoretical estimate. The symmetry of the neutron scattering operator implies a selection rule.

Tunnel splittings in solid CD_{4} estimated from heat capacity data
View Description Hide DescriptionHeat capacitymeasurements have been made on CD_{4}, isotopically purified by gas chromatography, in the temperature region 0.15–4.0 K. For T<0.5 K, the heat capacity is found to increase with decreasing temperature by much more than can be accounted for by contributions from impurities such as CHD_{3}. An analysis shows that the results can be reproduced by an array of low‐lying tunneling states with maximum splitting of 0.042±0.010 K. States in this region have been predicted by theory and found recently by measurements of inelastic neutron scattering. Enhanced thermal relaxation is observed in the calorimetric measurements in the region T<0.25 K. The onset of conversion between nuclear spin symmetry species is a possible cause.

Collisional ionization of atomic cesium and potassium by CH_{3}CN
View Description Hide DescriptionCollisional ionization of ground state Cs and K atoms by CH_{3}CN has been investigated in a crossed‐beam experiment for laboratory alkali kinetic energies from 26.5 to 90.7 eV (Cs) and 13.1 to 37.6 eV (K). The kinetic energies and angular distributions of product postive (Cs^{+} and K^{K+}) and negative (CN^{−}) ions were measured. From these, estimates of the internal energy of the molecular fragments were made. No evidence for production of a stable CN_{3}CN^{−} ion through alkali collisions was obtained.

Primitive model electrolytes. I. Grand canonical Monte Carlo computations
View Description Hide DescriptionMonte Carlo calculations in the grand canonical ensemble are described for coulombic systems, and carried out for 1:1, 2:2, 2:1, and 3:1 aqueous electrolytes in the primitive model with equal ion sizes. Energies and activity coefficients are obtained, and the scope and reliability of the method is discussed.

Primitive model electrolytes. II. The symmetrical electrolyte
View Description Hide DescriptionCanonical Monte Carlo results are reported for the 2:2 aqueous electrolyte in the primitive model. The various theories of dilute coulombic systems are compared with canonical and grand canonical Monte Carlo data on the thermodynamics and structure of this system: HNC, BGY, MPB, and ORPA+B_{2} are comparably successful, most other theories less so. The MC results show that there are many triple ions or larger clusters even at low concentrations. The problem of extrapolating experimental thermodynamic data for 2:2 electrolytes is discussed.